The present invention relates to a magnetic disk drive spindle motor and, more particularly, to a magnetic disk drive spindle motor structure for shielding the internal space of a motor from the outside.
In a magnetic disk drive spindle motor of this type, generally, dust such as a grease oil leaking from a motor or bearing scatters outside the motor and adheres to a magnetic storage medium to cause a serious accident such as a head crash. In order to solve this problem, conventionally, a magnetic disk drive spindle motor having a structure in which a magnetic fluid seal shields the internal space of the motor from the outside is used.
FIG. 7 shows a conventional magnetic disk drive spindle motor having the above-described structure. Referring to FIG. 7, reference numeral 1 denotes a spindle hub for stacking and supporting an information storage medium (not shown) outside. A permanent magnet 4 and a yoke 3 are fitted inside the spindle hub 1 to constitute a rotary drive system.
A pair of upper and lower rolling bearings 2 rotatably support the spindle hub 1 by the outer rings. A shaft 7 supports the inner rings of the rolling bearings 2 and fixes a coil 5 to oppose the permanent magnet 4 and the yoke 3, both of which are mounted on the spindle hub 1. A housing 6 fixes and supports one end of the shaft 7. A pair of bearing holders 15 hold the outer rings of the rolling bearings 2 and rotate together with the spindle hub 1. A pair of magnetic fluid seals 16 are provided to the bearing holders 15.
Each magnetic seal 16 is constituted by mounting a pair of magnetized doughnut-like disks onto the bearing holder 15 of the spindle hub 1 side with a small gap therebetween and injecting a magnetic fluid into the gap. The gap between the inner circumference of the disk and the shaft 7 is set at, e.g., 0.05 mm, and the gap is filled with the viscous magnetic fluid. The internal space of the spindle motor is stably shielded from the external space during the rotation and stop of the spindle hub.
In the above-described conventional magnetic disk drive spindle motor, the magnetic fluid seal 16 is very stable during the rotation and stop of the spindle hub 1 so that it effectively shields the internal space of the motor from the outside. When a load on the rotary body is, however, increased, a large load torque is required, because a resistance is generated by the magnetic fluid between the shaft 7 and the rotary body constituted by the spindle hub 1 or the like. Particularly, in a recent magnetic disk drive, operations at a higher speed are required, and the rated speed of the motor needs to be increased. The load caused by the magnetic fluid seal is greatly increased in proportion to the rotational speed of the motor.
In addition, the magnetic fluid scattered due to the pressure difference between the internal and external spaces of the motor causes an accident such as a head crash in the magnetic disk drive to degrade reliability. More specifically, the spindle hub 1 must be rotated at a high speed of several thousands rpm because such a magnetic disk drive spindle motor write/read-accesses the magnetic disk at a high speed. When a rated current is continuously supplied to the coil 5 to rotate the spindle motor at high speed, heat is generated in the coil 5 to expand the air inside the motor. When the internal pressure is increased, and the pressure difference between the inside and the outside exceeds the pressure limit, the magnetic fluid scatters to cause a head crash or the like.
An improved structure using the magnetic fluid seal is proposed in Japanese Patent Laid-Open No. 3-41693 in which an air path is formed at the upper portion of the shaft to couple the internal space of the motor with the external space, and a pressure regulating member is provided in the air path to adjust the pressure between the internal and external spaces. In this structure, however, the most advanced high-precision machining technique is required to form the air path inside the shaft, and number of machining steps undesirably increases. In addition, the air path formed inside the shaft impairs rigidity of the shaft to degrade stability of the shaft during the high-speed rotation.
On the other hand, a structure using no magnetic fluid seal is proposed in Japanese Patent Laid-Open No. 63-317977 in which a high-speed airflow is generated in a gap between a shielded bearing and a washer to shield the internal space of a motor from the outside. Another structure is illustrated in FIG. 2 of Japanese Patent Laid-Open No. 63-211167 in which a labyrinth mechanism is formed between the lower surface of a spindle hub and the upper surface of a housing.
In the former structure, however, the cost of the motor is increased because a specific member such as the shielded bearing is used. There is also a problem of reliability caused by a deterioration of the shield. On the other hand, in the latter structure, highly precise three-dimensional patterns are formed on the spindle hub and the housing. As in the above-described case, the most advanced high-precision machining technique is required, and an increase in number of steps causes an increase in manufacturing cost.